Fatal error: Tracker::get_recently_viewed()

Fatal error: Tracker::get_recently_viewed(): The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "BreadCrumbStack" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition in /var/www/html/crm/modules/Trackers/Tracker.php on line 109